Pioneering the use of nanoseismics as a site investigation tool

Field evidence shows that faults and fractures can act as focused pathways or barriers for fluid migration. As a result, detailed imaging, i.e. location, orientation and hydraulic properties, of fracture networks not visible at the surface, is crucial for modern engineering applications, such as CO2 storage, geothermal energy exploitation, tunnelling. To-date, such information is not available at the required level of detail and accuracy. Currently, methods for the imaging of fractures at a resolution of less than 100m are based on seismic reflection/refraction and surface waves. These methods require a large number of sensors (usually over 100 making their application costly) and their success highly depends on the elastic properties of the rock and the inclination (dip) of the underlying strata. The fractures can be identified at a maximum depth of 100 m below ground surface.

The main aim of this project is to (1) develop a new site investigation tool that utilises micro-seismicity for the identification of individual fractures that accommodate flow at depths down to 4km and (2) build the skills and knowledge necessary to implement it within industrial applications directly related to fluid flow systems at depth. This will result in the publication of a new protocol for microseismic borehole stimulation to image fractures that accommodate flow over large spatial scales.

A new monitoring technology will be used and further extended: short-period seismometers with the combination of "Hypoline" software. The latter can help distinguish between weak signals and noise and reduces the signal-to-noise ratio to 1:1. The combination of sensors and software is known as nanoseismics (NS). Although new and very promising, this technique has a limitation: it can provide a location, time of origin and magnitude for the recorded events, however, its does not provide an estimation of the location error, nor the moment tensor that would allow determination of fracture orientation, two very important parameters for providing a detailed image for engineering applications. This project will pursue to further extend this capability for the determination of individual fracture orientations and quantification of fracture permeability.

There is a unique opportunity to found both the testing of the efficiency of NS as a site investigation tool, and the development of the analysis technique, by recording data at an existing site with high potential to act as geothermal resource in NE England. Permission has been granted to conduct a series of injection tests at a borehole on the site in order to record the induced microseismicity and locate and image the fractures that accommodate the flow at a depth of up to 2km.
The outcomes of this research will contribute to a better understanding of sub-surface processes through characterisation of heterogeneous flowing fracture networks over large spatial scales.

A unique opportunity has risen to develop nanoseismics (NS) as a site investigation tool based on the knowledge obtained from the study of a natural system: Pytharouli et al. (2011) concluded that detailed seismic data sets can be used to accurately identify fractures at depth. The proposed research aims to provide detailed insights in the mechanical behavior of rocks and highlight individual fluid flow pathways for the optimisation of a geothermal exploitation system. We will use NS as a new engineering tool for the optimisation of engineering design and risk control that could be integrated into other systems, e.g. in tunnels for the characterisation of the engineering damage zone. The beneficiaries of this study include:
1.Public Sector: This project will impact the nation's health and wealth; it will contribute to the economic competitiveness of the UK with the potential to apply the technology to monitoring the mechanical evolution of the rock mass surrounding gas storage, CO2 sequestration sites and underground waste repositories. The latter is of major importance, i.e. in the nuclear power industry, specifically after the Fukushima disaster in 2011. The reduction of the uncertainties involved in the geological disposal of nuclear waste can positively influence the public attitude towards the use of nuclear power/carbon capture as well as increase regulatory confidence, e.g. for safety case development and in the case of CO2 storage, for monitoring leakage.
2.Industry: The monitoring of underground repositories could form an important element of site characterisation and long-term monitoring for geological disposal of nuclear waste and carbon capture projects and consequently improve the regulatory control. This is a beneficial tool for industry that will contribute to the UK's successful progression to a low-carbon economy.
The outcomes will attract further funding in the form of R&D investment from international businesses and industries related to oil and gas, geothermal energy exploitation etc. The results will be used as proof of concept for the ability of the proposed methodology and equipment to enhance the efficiency and performance of businesses, e.g., maximise the geothermal capacity of underground reservoirs by providing a better understanding of the sub-surface flow system or being used as a supplementary tool for site investigation and consequently contribute to environmental sustainability.
The use of NS will increase the competitiveness of the UK and worldwide engineers, as a new powerful monitoring tool for site investigation. Additionally, it will enhance the ability to detect weak precursor signals of a potential failure and distinguish these among the recordings from noise sources.
3.Additional benefits: The short period seismometers, being portable, easily transferred and providing a visual inspection of the recorded signals in real-time, can be used at schools to contribute to engineering/seismology related projects and enhance the learning process of the pupils.
In the longer-term, a micro-seismic system could also be developed for public hazard monitoring, such as rainfall-induced landslides and rock-falls. The former is the main cause of slope instability in the UK and worldwide. The knowledge of the location of potential flow pathways can help identify potential risk areas and lead to the development of a new form of early warning system for the real-time detection of rockfalls/landslides, thus increasing the safety and reliability of public transportation and indirectly enhancing the quality of life for the public and increase the feeling of safety.
The outcomes will provide necessary information to policy-makers for natural hazard management; results will help identify failure mechanisms, develop control measures and minimize the impact of a potential failure, especially in urban areas, and the cost of unnecessary precautionary measures by better understanding of the risks involved.